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142

Revolutions of Lathe Mandrel.

variation, for the linear velocity at the surface of the work must j
be constant. Fig. 133 shews that if ab and ajbl are equal, the I
angle a^cb, must be greater than acb.

Let r=radius of Tvork in inches.

V — speed of cut in feet per minute.

N = revolutions per minute to produce V.

Then,

12

and,

And as the''cutting,.sgejgds for roughing are, say:—

For brass...........................    60 feet per min.       ""J

For gun metal   ....................    50   „       ,,              S^

For cast iron.......................    40   „       ,,             <,

For wrought iron..................    40   „       ,,             ^

For steel..............................    30   „       „             .g

We have:—                                                                         ^

Revolutions per m. for brass......... =* 115-rad. in ins, :§»

„              „          gun metal ... =   95-        „        ^

„              „          cast iron   ... =    76-        „         -3

„              „          wrought iron =76-        „         *

,,              „          steel ......... =   57 ~       „        S

To effect this variation without altering the angular velocity of
the main shaft, cone pulleys and back gear are employed.

The cone pulley c is driven by a belt, from a like pulley on
the countershaft overhead, but tbe latter is reversed end for end,
so that its small diameter is opposite the large diameter on the
headstock. As the sum of driving and driven pulley diameters
is constant, the belt will fit any pair, and a change of velocity will
be effected, the highest being due to the smallest pulley on the
hejoVstock. (See Fig. 539,, p. 535.)

But as sufficient variation cannot thus be obtained we use the
jsgur "wheels knowiijas back gear. The mandrelD (Figs. 131 and
i34)*ls attached directly" to "the work by a driver. But the cone
pulley runs loose upon the mandrel. Deferring to Pig. 134, the
bolt E serves to connect the pulley with the wheel F, which is
keyed to D, and by sliding i radially outward till it engages
between lugs G an the pulley, F and c are united, and the
mandrel is driven cKrectly.

Slower speeds are obtained by releasing E, and allowing the